1. Field of the Invention
The present invention relates, for example, to a method for fabricating a thin film transistor. More particularly, it relates, for example, to a method for fabricating a thin film transistor that includes forming a metal catalyst layer of a predetermined composition on a substrate.
2. Description of the Related Art
Generally, thin film transistors (TFTs) used for flat display devices are fabricated as follows. Amorphous silicon is deposited on a transparent substrate such as glass or quartz. The amorphous silicon is then dehydrated and impurity ions for forming a channel are implanted. Subsequently, the amorphous silicon is crystallized to form a semiconductor layer. A gate insulating layer and a gate electrode are formed. Next, an ion implantation process is performed to form source and drain regions. Then an interlayer insulating layer and source and drain electrodes are formed.
The method for crystallizing the amorphous silicon into the polysilicon may include solid phase crystallization, excimer laser crystallization, metal induced crystallization, and metal induced lateral crystallization. The solid phase crystallization method anneals an amorphous silicon layer for several hours to tens of hours at a temperature of about 700° C. or less (the limiting temperature is typically the deformation temperature of the substrate material). The excimer laser crystallization method irradiates an excimer laser onto a silicon layer to locally heat the silicon layer to be crystallized for an extremely short time at a high temperature.
The metal induced crystallization method is a method in which a metal such as nickel, palladium, gold, or aluminum is implanted into or in contact with the amorphous silicon layer to induce the amorphous silicon to go through a phase change into polysilicon. The metal induced lateral crystallization method crystallizes the silicon layer by having silicide obtained from reaction between the metal and the silicon laterally extended to induce the sequential crystallization of silicon.
However, the solid phase crystallization method has a disadvantage that severe deformation of the substrate may occur when the substrate is annealed for a long time at a high temperature, and the excimer laser crystallization method has disadvantages that not only high-cost equipment and high maintenance cost are required but also the surface roughness of the polycrystalline silicon layer is poor. In metal induced crystallization and metal induced lateral crystallization, a metal material that induced the crystallization remains in the polycrystalline silicon layer and can increase leakage current in the semiconductor layer.
In order to solve the above-described disadvantages, a method of forming a capping layer on an amorphous silicon layer and forming a metal catalyst as a very thin metal catalyst layer by means of sputtering, and annealing it for crystallization is disclosed (Korean Patent Publication No. 2003-0060403), however, there exists a disadvantage that the metal catalyst layer should be formed to be extremely thin, that is, it should be formed with extremely low density.